"Native Clay" ceramic slip available as a clay source?

When I was a child, we used to live in Alaska (Anchorage) and at that time my mother used to do a lot of ceramics. We used to be able to buy slip at the local ceramic store that had Ptarmigan (sp) bay clay in it as well as the traditional slip - the result when fired was that it made a unique swirls of white (the regular slip) and red (the native clay out of the bay). It has been a LONG time since I have thought much about this stuff, and I don't imagine it is affordable to have the slip shipped down to me if it is available - but would there be any clay from Laguna or other clay source that would provide a similair effect?

I wish I had an example of what "native clay" as it was called looked like after it was fired, but I don't and couldn't locate anything on the internet - is there anyone out there that knows what I am talking about - and would they make anything like that in traditional clay medium and not slip?

Back to my original question - I found this picture by accident on the internet when searching for "western theme"....but it was listed as "terra cotta" pottery.....how would a person do this? OR, is it a mix of slip as I was originally asking about Native Clay (mixing Alaska Native Clay with regular slip)...
thank you all!

I've done something similar by wedging two different clays together slightly, then building my piece. The finished piece has to be scraped or the surface layer removed some other way because the two clays get mixed together by the building process, but if the two clays are compatible, you get that sort of wild swirly surface. If they're NOT compatible, well, bad things happen. I used two different Laguna clays, but you can do the same thing by mixing colorants into one bunch, then mixing the two together. I did it hand-building (coils) but I don't see why you couldn't do it with slips if you're casting.